Upgrade from old version (6) to new one

How can we upgrade our old system version 6 to new one without loosing anything of our database and in the same times all customized solutions in old version implement in new version
Many thanks.

update to major release ex. v12 → v13
nodejs >= 10 required (u can use 12)

cd ~ 
curl -sL https://deb.nodesource.com/setup_12.x -o nodesource_setup.sh 
sudo bash nodesource_setup.sh 
sudo apt install nodejs nodejs -v 

now upgrade

cd ~/frappe-bench bench switch-to-branch version-13-beta frappe erpnext --upgrade 
bench update --patch 
bench migrate 
sudo bench --site <my site> migrate
bench update --build
bench clear-cache-
bench website-clear-cache

sudo supervisorctl reload
sudo service nginx reload

if required upgrade Venv

sudo -H pip3 install --upgrade pip
bench migrate-env python3
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